The analysis of complex mixtures often requires more than one separation process in order to resolve all the components present in a sample. It is for this reason that two dimensional (2D) separation schemes have been devised. When constructing a successful 2D system several criteria need to be addressed. Firstly, the two techniques should base their respective separations on as different a means as possible. Doing so will reduce the amount of redundant information contained in the 2D dataset. Secondly, the second dimension needs to operate at a sufficient rate of speed so that frequent "sampling" of the first dimension is achieved. This is necessary to ensure that the peak profiles obtained from the first separation dimension are not degraded. See Giddings, Analytical Chem. 56, 1258A (1984). Bushey and Jorgenson realized that capillary zone electrophoresis (CZE) is well suited as a second dimension separation due to the ability to obtain highly efficient separations with short run times. In 1990 they introduced the first fully automated 2D system, which coupled column liquid chromatography with CZE. See U.S. Pat. No. 5,131,998 (the disclosure of which is to be incorporated by reference), and Bushey and Jorgenson, Analytical Chem. 62, 978 (1990). In this system all sample components separated in the liquid chromatography dimension were subjected to separation by CZE (although the entire liquid chromatography elution volume was not reinjected for CZE). The coupling of the two dimensions was accomplished using conventional column and sample loop (valve loop system) technology.
A 2D system that coupled microcolumn liquid chromatography with CZE would be microscale, able to perform complete analyses with only a few hundred nanoliters of sample. Microcolumns (columns with an inner diameter .ltoreq.250 .mu.m) have been shown to be considerably more efficient than conventional size columns. The relative merits of using microcolumns rather than conventional columns has been previously discussed. See Kennedy and Jorgenson, J. Microcolumn Separations 2,120 (1990). However, microcolumns and their inherently small column volumes and operating flow rates pose an engineering problem in terms of designing a suitable interface for use with CZE, and it has not heretofore been suggested how microcolumn liquid chromatography might be coupled to CZE to provide a 2D chromatography system.